Polyolefins produced in a high pressure (HP) process are widely used in demanding polymer applications where the polymers must meet high mechanical and/or electrical requirements. For instance in power cable applications, particularly in medium voltage (MV) and especially in high voltage (HV) and extra high voltage (EHV) cable applications, the electrical properties of the polymer composition used in the cable has significant importance. Furthermore, the electrical properties of importance may differ in different cable applications, as is the case between alternating current (AC) and direct current (DC) cable applications.
A typical power cable comprises a conductor surrounded, at least, by an inner semiconductive layer, an insulation layer and an outer semiconductive layer, in that order. The cables are commonly produced by extruding the layers on a conductor.
The polymer material in one or more of said layers is often cross-linked to improve e.g. heat and deformation resistance, creep properties, mechanical strength, chemical resistance and abrasion resistance. During the crosslinking reaction, crosslinks (bridges) are primarily formed. Crosslinking can be effected using e.g. a free radical generating compound which are typically incorporated into the layer material prior to the extrusion of the layer(s) on a conductor. After formation of the layered cable, the cable is then subjected to a crosslinking step to initiate the radical formation and thereby crosslinking reaction.
Peroxides are very commonly used as free radical generating compounds. The resulting decomposition products of peroxides may include volatile by-products which are often undesired, since e.g. may have a negative influence on the electrical properties of the cable. Therefore the volatile decomposition products such as methane are conventionally reduced to a minimum or removed after crosslinking and cooling step. Such removal step, generally known as a degassing step, is time and energy consuming causing extra costs. It will be appreciated that a cross-linked polyethylene material is thermosetting.
LDPE is also an ideal cable forming material from a cleanliness point of view. LDPE can be manufactured in very pure form without impurities. In contrast low pressure polymers often contain more gels and catalyst residues which can lead to defects in the cable.
In order to increase the power transmission capability of extruded high voltage direct current (HVDC) cables, the voltage needs to be increased. In HVDC cables, the insulation is heated by the leakage current. The heating is proportional to the insulation conductivity×voltage2. Thus, if the voltage is increased, more heat will be generated. This may lead to thermal runaway followed by electric breakdown. Thus, in order to increase the power transmission capacity, insulation material with very low electrical conductivity is needed. In one embodiment, the voltage may be increased from today's highest level of 320 kV to 640 kV or more.
The present inventors have now investigated the possibility of reducing conductivity though combination of the LDPE with a secondary polymer. The secondary polymer however is typically one made using an olefin polymerisation catalyst and hence catalyst residue content might be high. This leads to a greater risk of mechanical breakdown compared to pure XLPE. Nevertheless, the inventors have surprisingly found that the combination of low amounts of HDPE and LDPE leads to remarkable conductivity reduction in thermoplastic and cross-linked insulation layers even at very low levels of HDPE.
Thermoplastic LDPE offers several advantages as cable insulation compared to a thermosetting cross-linked PE. As the polymer is not cross-linked, there is no possibility of peroxide initiated scorch. In addition, no degassing step is required to remove peroxide decomposition products. The elimination of crosslinking and degassing steps can lead to faster, less complicated and more cost effective cable production. However, the absence of a cross-linked material can lead to a reduced temperature resistance and hence significant problems with creep. Thus, better thermomechanical properties are needed in order to provide a polymer material that can be used without crosslinking in a cable insulation layer.
The present inventors have now found that the combination of an LDPE with a low amount of an HDPE can provide a blend such as a thermoplastic blend which is ideally suited for cable manufacture. Surprisingly, these blends have much lower conductivity than the corresponding LDPE alone and do not suffer from dielectric breakdown. Moreover, as the HDPE content is so low, this leads to a reduced risk of mechanical breakdown caused by the presence of the less pure HDPE with the LDPE.
Moreover, the inventors have found that certain LDPEs can be combined with low amounts of HDPE to form a blend which has remarkably low conductivity in cross-linked form.
The LDPE of use in the invention is not itself new and it has been previously proposed in the literature. Moreover, the possibility of using non cross-linked LDPE in the insulation layer of a cable is not new. In WO2011/113685, LDPE of density 922 kg/m3 and MFR2 1.90 g/10 min is suggested for use in the insulation layer of a cable. WO2011/113685 also suggests using other polymers individually in the non cross-linked insulation layer of a cable.
In WO2011/113686, a blend of LDPE and HDPE is used to manufacture a cross-linked polymer composition that can be used in the insulation layer of a cable however the amounts of HDPE taught are relatively high (minimum 5 wt %).
In US2013/0175068 there is a disclosure of the use of HDPE and LDPE to improve breakdown strength in thermoplastic cables. 20 wt % HDPE is exemplified in the examples.
The blends of the invention are therefore ideal for use in the insulation layer in a direct current (DC) power cable or AC power cable and the blends enable cables that operate at voltages higher than possible today.